1. Field of the Invention
The subject invention pertains to the art of electrically controlled printers and more specifically to thermal printing systems in which the electric current supplied to thermally responsive elements is varied according to the printing history.
2. Description of the Prior Art
In the development of thermal printing technology, it has become possible to perform high speed printing of characters and graphics. Generally, in thermal printing technology a plurality of electrically responsive thermal printing elements, for example, electrically resistive elements, are responsive to a print pulse which is an input electrical current pulse which causes the resistive element to generate heat. The heated resistive printing element, in contact with paper having a thermal dye impregnated thereon will cause a dot to appear on the paper. The printing elements may be configured in any manner desired to provide alphabetic or numeric characters or graphics.
With the need for the high speed printing of present day information systems, it has become necessary to control the print pulse to achieve quality displays. In order to achieve fast thermal printing, printheads have been produced utilizing on-head integrated circuit shift registers and drivers which reduce the overall time required to address a head with a random dot pattern. Prior to this integration, groups of printing elements typically were multiplexed in five or more print intervals. Alternatively, each element could be brought out to a separate electrical lead for interconnection. A typical printhead has elements arranged in a 1 by 256 dot configuration. It therefore became obvious that direct interconnection is not desirable since 256 separate electrical leads would be needed to the printhead.
The use of serial shifthead drivers in the printhead permits connection of fewer leads to the printhead. Fast operation is achieved by enabling the loading of the shift register at speeds of typically 1 megahertz for the on/off pattern of the serial entered dot on the data input line to the shift register. At the conclusion of the shift, for example after the inputting of 256 bits to the shift register, the pattern is present at the output of a parallel device, which pattern is then applied to the printing elements. Thus, users were able to print all the dots in a single print pulse. That is, the entire dot pattern comprised of signals for 256 dots existing at the output of the shift register would be printed upon the application of a single print pulse which enabled the transfer of the electrical signals from the output of the parallel device ultimately to the printing elements.
A problem however arose as a result of the new printing technique. When one of the individual printing elements were energized in several consecutive or nearly consecutive print cycles the elements became overheated. This would result in a blushing or blooming of the paper dye beyond the desired printing area rendering the printed copy unclear and also often resulted in the eventual thermal overloading and destruction of the printing elements themselves. This occurred since the thermal response of the individual elements is not instantaneous. Typically, sufficient temperature for a reaction with the paper dye is achieved at the printing element surface after a short power pulse in the range of 1 to 10 milliseconds. A longer period is required for the element to return the printing element substrate to ambient temperature, this decay period being typicaly several times the initial response. If the element is reenergized at a point shortly after it has reached its peak temperature but before it has had the chance to return to the ambient temperature of the substrate, due to residual heat, a higher peak temperature will be reached in the element than was previously obtained.
If the printing element is continually reenergized in this manner, eventual cumulative temperature rise will cause paper blushing and eventually element destruction from overheating. If however the element is reenergized after it has returned to the ambient temperature of the substrate or a temperature relatively close to that point, minimal cumulative heating occurs.
Users of the integrated drive heads described herein who encountered this problem have attempted to overcome it by using one of two solutions. (1) The thermal response time of the element has been minimized, to increase the rate at which the element dissipates heat and thereby decrease the time it takes the element to return to ambient temperature. Minimization of thermal response time also provides faster heating of the element. In present thermal printheads this has been optimized to the state of the art and the response times achieved are still not adequate to prevent the problems of overheating and dot blushing. (2) Faster cooling of the elements has been provided for the rapid dissipation of heat. This is ordinarily done by bonding a massive heat sink or radiator to the printhead substrate. This technique is wasteful of power, since, a good deal of heat generated in the printhead is radiated into the environment and is thus not available to be utilized in the printing process. Furthermore an idle period during which no printing could take place is still required to allow for the proper heat conduction into the heat sink. The enforced idle period slows printing.
It is thus an object of the invention to provide an electrically responsive printer capable of printing at high speeds;
It is a further object of the invention to provide a high speed thermal printer;
It is another object of the invention to provide a high speed thermal printer capable of power efficient operation which does not dissipate large amounts of heat into the environment;
It is still a further object of the invention to provide a thermal printer without an enforced idle period;
It is a further object of the invention to provide a high speed thermal printer in which the individual printing elements do not overheat during the printing process; and
It is yet another object of the invention to provide in a high speed thermal printer a high quality printout.
These and other objects and advantages of the invention will become more apparent upon reference to the specification and the drawings annexed hereto.